Vertical Momentum Transport by Internal Gravity Waves Above the Equatorial Undercurrent at 140°W

Abstract

Abstract Strong vertical shears occur in the upper Equatorial Ocean as the trade winds drive the South Equatorial Current westward above the eastward flowing Equatorial Undercurrent. An extremely large “effective viscosity” or vertical momentum transport is required to maintain the speed‐differential between the currents as observed. In the 2012 EquatorMix Experiment data from a 1.8 km optical fiber temperature array and a surface scattering radar were combined with high‐resolution shipboard profiling CTD and Doppler sonar measurements to determine the directionality of energetic ∼600 m wavelength internal waves existing above the Undercurrent. A large vertical momentum flux is found (∼10−4 m2 s−2), with waves excited by nocturnal sea surface convection and maintained by near‐surface critical layer over‐reflection. The net downward‐westward momentum flux is an index of the energy lost during reflection below the Undercurrent. Together with near‐surface‐turbulence, these waves provide the momentum transport needed to balance the large‐scale forcing of the equatorial current system.

Keywords

• equatorial undercurrent
• wave momentum transport
• internal waves
• critical layers
• wave instability processes